(a) Field of the Invention
The present invention relates to a backlight unit having an inclined light emission surface and, more particularly, to a backlight unit having an inclined light emission surface and a plurality of lamps behind the light emission surface. The present invention also relates to a double-sided display device including the backlight unit.
(b) Description of the Related Art
A backlight unit is a surface-emission light source generally used in a LCD (liquid crystal display) device. The backlight unit has an emission surface on a scattering plate for scattering the light emitted by the lamps disposed therein. The backlight units are categorized into two types: an edge-lamp backlight unit including a pair of lamps disposed in the vicinities of both edges of the scattering plate; and a parallel-lamp backlight unit including a plurality of parallel lamps disposed behind the scattering plate. The parallel-lamp backlight unit has the advantages of a larger light emission area and an increased luminance of the light emission surface compared to the edge-lamp backlight unit. Patent Publication JP-A-2000-10094, for example, describes a parallel-lamp backlight unit.
A variety of LCD devices have been used in a variety of applications heretofore, including a double-sided LCD device having a pair of LCD panels on the front and rear sides thereof. The double-sided LCD device generally includes therein a double-sided backlight unit used in common for the front and rear screens. A double-sided, parallel-lamp backlight unit is described in JP-A-2000-338483, for example.
FIG. 7 shows an LCD device including a double-sided, parallel-lamp backlight unit (hereinafter, may be referred to simply as double-sided backlight unit) and described in JP-A-2000-338483. The double-sided backlight unit 200 has a pair of scattering plates 201 supported parallel to one another by a pair of support spacers 203, and a plurality of lamps 202 extending parallel to one another in a row or array on the central plane 204 between the scattering plates 201. The backlight unit 200 is associated with front and rear LCD panels 205 disposed on the respective scattering plates 201.
The number of parallel lamps 202 as well as the pitch “P” thereof is determined based on the luminance requested of the light emission surface of the backlight unit 200. The pitch “P” of the parallel lamps 202 is generally constant within the backlight unit 200 for achieving a uniform luminance over the emission surface.
The double-sided LCD device may be disposed in a train or pathway over the eyes of passengers or passersby for an advertisement by displaying a good etc. on both the front and rear screens. In such a case, these screens should be preferably inclined from the vertical planes to be directed slightly downward. On the contrary, the screens of the double-sided LCD device should be preferably inclined from the vertical planes to be directed slightly upward if the screens are disposed below the eyes of an observer.
It is noted that both the screens of the double-sided LCD device cannot be directed, for example, downward concurrently because both the screens are fabricated as an integral body.
FIG. 8 shows a conceivable double-sided LCD device including a pair of LCD panels 205 and a backlight unit 200a having front and rear scattering plates 201, which may be directed downward at the same time. In this structure of the backlight unit 200a, both the emission surfaces are inclined by angle +θ, with respect to the central plane 204 of the backlight unit 200a, in the opposite directions and thus disposed in symmetry with each other with respect to the central plane 204. The double-sided LCD device having the double-sided backlight unit 200a provides a suitable viewing angle for the observers observing the front and rear screens of the LCD device, if the LCD device is hanged from the ceiling, with the large-thickness side of the LCD device being disposed at the top side.
In the structure of the backlight unit 200a shown in FIG. 8, the distance between the scattering plate 201 and the lamps 202 depends on the location in the y-direction. It is to be noted that a smaller distance between the scattering plate 201 and the lamps 202 provides a higher luminance on the screen of the LCD panel 205. That is, the backlight unit 200a shown in FIG. 8 has a disadvantage in that the luminance varies along the y-direction on both the front and rear emission surfaces.
The above disadvantage does not arise in a single-sided backlight unit so long as the direction of the array of lamps is inclined in accordance with the inclined screen of the LCD device. However, as in the case of the LCD unit shown in FIG. 9, if there is a limitation due to the structure or arrangement of the LCD device such that the array of the lamps 202 in the backlight unit 200b cannot be inclined in accordance with the inclined screen of the LCD panel 205, a similar problem will arise in the case of the single-sided backlight unit 200b. 